Conventionally, a technique has been disclosed in which the motor drive is mainly controlled by configuring an inverter circuit from a power module on which power semiconductor chips are mounted and by performing a switching operation on direct-current power and thereby converting the direct-current power into alternating-current power (Patent Literature 1). An electric current instantaneously flows to only a few specific chips in the power module to generate heat. However, the heat-generating chips are instantaneously switched; therefore, usually, the chips generate heat equally. On the other hand, in particular, in driving of a servomotor, there are many cases, such as a case of holding a heavy object, where electric power is supplied to the motor without involving rotation of the motor. In such cases, an electric current concentratedly flows to a few specific chips in a module and the amount of heat generation locally increases. There is a demand for a cooling device having high heat radiation properties that can efficiently and quickly diffuse and radiate heat even in such cases.
As such a cooling device, conventionally, there is a cooling device in which a material having a high thermal conductivity is used to obtain a structure having high heat radiation properties. For example, in Patent Literature 1, a heat radiation plate is configured from two graphite layers. A high thermal conductivity is obtained in the horizontal direction in the first layer and obtained in the vertical direction in the second layer, thereby increasing the heat radiation properties.